This invention relates generally to the control of temperatures and selectivity in reactors that circulate catalyst. This invention also relates to processes for reactions that use a particulate catalyst as an oxygen carrier through reversible oxidation-reduction. This invention can also relate specifically to the production of acrylonitrile by the ammoxidation of propylene in the presence of a particulate catalyst.
Processes that contact reactants with circulating catalyst particles in a reaction zone are well known. One of the most well known processes that contacts reactants and regenerants with circulating catalyst particles is the fluidized catalytic cracking (FCC) process for the conversion of heavy hydrocarbons. U.S. Pat. No. 3,844,973 shows a regenerator arrangement used to regenerate catalyst in an FCC process that has a first dense bed that supplies catalyst to a relatively dilute phase catalyst mixture in a superadjacent transport riser. U.S. Pat. No. 3,919,115, U.S. Pat. No. 3,953,175, and U.S. Pat. No. 4,340,566 shows a variety of additional FCC regenerator arrangements that operate with a relatively dense phase bed and that supply catalyst particles to a relatively dilute phase transport riser.
Processes for the ammoxidation of propylene to produce acrylonitrile are generally well known. U.S. Pat. No. 4,246,191 provides an extensive list of references and specific descriptions of various patents that describe different methods of contacting reactants for the production of acrylonitrile with particulate catalyst for carrying out the ammoxidation reaction. U.S. Pat. No. 4,246,191 is particularly directed to temperature control of particulate catalyst in a fixed bed reaction zone having a top bed surface that extends close to the inlets of devices for the separation of catalyst from the acrylonitrile products. Control of the temperature in the particle bed minimizes the temperature deviation along the reactor profile.
The ammoxidation of propene to produce acrylonitrile is generally believed to be a redox process and that lattice oxygen from circulating solids can supply oxygen to the reaction. It is also known that lattice oxygen can be regenerated by air at certain temperature ranges. U.S. Pat. No. 4,152,393 discloses a reactor design for circulating catalyst from a first fluidized bed for the ammoxidation of propylene to produce acrylonitrile and a second bed for the regeneration of the catalyst. The arrangement continuously circulates catalyst from one bed to another and isolates the fluids from each individual bed to prevent intermixing. U.S. Pat. No. 4,246,192 teaches the oxidative regeneration of catalyst for the ammoxidation of olefins by the taking of a small stream of catalyst from a reaction zone for the ammoxidation of olefins. In this arrangement catalyst transfer lines connect the regeneration zone and reaction zones in this arrangement.
The ammoxidation reaction may also be carried out without a separate regeneration zone in which case oxidation of the catalyst material occurs within the reaction zone. Typically, a stoichiometric excess of oxygen in relation to the feed gas will be sent to the ammoxidation reaction zone.
Single bed reactors for the ammoxidation of propylene to produce acrylonitrile have been generally preferred. In particular the fixed bubbling bed type have been preferred to reduce the volume of catalyst circulation required in conventional transport-type arrangements for ammoxidation of propylene (see page 14 of xe2x80x9cNew Developments in Selective Oxidation IIxe2x80x9d, Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, Sep. 20-24, 1993). The use of multiple beds of the same type, again the bubbling bed variety, is disclosed in a paper by the Chemische Technik 48 (1996) entitled xe2x80x9cEffect of Hydrodynamic Conditions in an Industrial Scale Fluidized Bed Reactor on Selectivity and Yield in Acrylonitrile Preparation by Ammoxidation of Propenexe2x80x9d.
The ammoxidation of propene typifies heterogeneous catalytic reactions that evolve large amounts of heat, but require a relatively narrow range of temperature to be maintained as the reactants contact the particulate catalyst in order to inhibit secondary reactions that produce unwanted by-products. In the case of ammoxidation the reaction is very exothermic and continued reaction can further oxidize the product into nitrogen and carbon oxides when in the presence of oxygen. As demonstrated by the prior art, reactors for ammoxidation of propylene to produce acrylonitrile are well known. The current reactor configuration favored for most commercial arrangements is a bubble fluidized-bed reactor. The superior heat transfer characteristics of the bubble fluidized-bed reactors have led to their wide spread acceptance for ammoxidation. The reactor removes the reaction heat from the ammoxidation reaction efficiently by installing a heat transfer device inside the reactor.
A long standing problem with the use of a bubble fluidized-bed reactor is the relatively low superficial gas velocity that can be maintained through the reactor while still operating in a dense phase condition. Superficial gas velocity through the bed must be restricted to around 0.5 m/s. These low velocities present serious back-mixing problems within the reactor that randomly changes residence time and overall lowers the selectivity of the process to the desired product.
One solution for reducing the degree of backmixing has been a circulating fluidized bed (CFB) reactor. This reactor reduces the degree of gas mixing by passing reactants and catalysts to a transport conduit such as a riser-type reactor. Such reaction arrangements promote careful control of reaction times and generally approach a plug-flow contacting of reactants and catalyst particles. A draw back to the CFB-type reaction arrangement is the removal of heat. Heat transfer devices are typically not suitable for use within the circulating particulate catalyst environment. The provision of a heat exchange surface in a CFB-type reaction zone also leads to catalyst attrition as well as presenting poor heat transfer conditions due to the relatively low density of the catalyst contained therein. Therefore, reaction heat has to be removed from the reactor by circulating catalyst to avoid any large temperature increase which, again, will result in a higher yield of undesired by-products such as carbon dioxide. Typically, such reactors require a high catalyst circulation rate to maintain low temperature increases in the reactor. For example, a typical CFB-type reactor must circulate about 800 kg of solid catalyst for each kg of acrylonitrile that is produced to keep the temperature increase below about 10xc2x0 C. as the catalyst and reactant mixture passes from the inlet to the outlet.
It is known from EP 0 189 261 A1 to use the combination of a bed-type reaction zone below a riser-type reaction zone for the production of maleic anhydride. EP 0 189 261 A1 also discloses that a metal oxide may serve as a carrier for a significant portion of the stoichiometrically required oxygen through rapid and reversible oxidation-reduction. However, EP 0 189 261 A1 teaches, in conjunction with its examples for maleic anhydride production, that riser reaction zones, fluidized bed reaction zones, and combinations thereof have no particular advantage over each other.
Therefore, it is an object of this invention to increase selectivity for acrylonitrile by reducing the backmixing inside a particle contact-type reactor.
It is another object of this invention to lower catalyst circulation in a CFB-type reactor arrangement for the production of acrylonitrile.
It is a further object of this invention to control oxidation reactions that take place within a typical ammoxidation reaction.
It is a yet further object of this invention to improve the selectivity of acrylonitrile generated from an ammoxidation reaction.
It is another object of this invention to improve the mixing of oxygen that enters the ammoxidation reaction zone.
These objectives are achieved by a hybrid reactor arrangement that provides dual reaction zones which simultaneously increase the acrylonitrile yield and reduce the production of undesired by-products by improving the selectivity of the process to acrylonitrile production and lowering the catalyst circulation rate. The hybrid reactor has a bubbling fluidized-bed reactor in one section and a circulating fluidized-bed reactor in another section. The hybrid reactor efficiently removes heat in the bubbling fluidized-bed reactor section and completes the final conversion of reactants in a CFB reaction zone that reduces backmixing. Further selectivity and temperature control may be provided by staging the introduction of an oxygen-containing reactant into the CFB reaction zone. Together the two reaction zones improve the selectivity of acrylonitrile from the ammoxidation reaction.
This invention provides the necessary temperature control for sustaining the desired selectivity to production of acrylonitrile in the ammoxidation process. To achieve high selectivity the temperature must be high enough to sustain activation of the material and dispersion of oxygen from the lattice of the oxygen-supplying material. At the same time, temperature is often restricted to prevent molecular oxygen from producing unwanted by-products by reaction with reactants or products.
As a result, this invention combines the best of a conventional bubbling fluidized-bed with a CFB bed reactor to improve yield and selectivity while reducing the required catalyst circulation rate. The bubbling fluidized-bed contains a heat transfer device that removes the reaction heat from the bottom and maintains isothermal or nearly isothermal conditions within the bubbling fluidized-bed reactor. On average, the reaction zone of this invention can raise the yield of acrylonitrile by 5% or more from that obtained by complete bubbling fluidized-bed reactor when operating at a propylene conversion rate of about 98% in an ammoxidation reaction for the production of acrylonitrile. At equivalent conversions and selectivity, the hybrid reactor of this invention can reduce catalyst circulation by about ten-fold over that required to obtain similar yields from a CFB reactor arrangement.
The flow regimes within the different portions of the hybrid reactor are important elements of this invention. Fluidized conditions within the bubbling fluidized-bed portion of the reactor typically include a catalyst holdup of at least 25% and a superficial velocity that normally does not exceed 1 meter per second. Catalyst holdup is the ratio of the bulk volume of the catalyst contained in a vessel to the volume of that vessel. A majority of the initial conversion of the entering hydrocarbon feed will occur in the bubbling fluidized-bed portion of the reactor and, more typically, from about 60 to 70% of a conversion of propylene will be achieved in the bubbling fluidized-bed portion of the reactor. During operation, it is also important that the bubbling fluidized-bed temperature be carefully controlled. Typically, indirect heat exchange with an appropriate cooling fluid will maintain the bubbling fluidized-bed portion of the reactor at the desired temperature.
After initial reaction in the bubbling fluidized-bed portion of the hybrid reactor, reactants will pass from the bubbling fluidized-bed portion into the CFB portion of the hybrid reactor at a temperature controlled by the prior dense bed heat exchange and reaction. In most arrangements, the bubbling fluidized-bed is located below the CFB reaction section and the catalyst and reactants move upwardly through a riser-type transport conduit to achieve another 30 to 40% of the reaction under reaction conditions that approach plug-flow contacting. In a typical flow regime for the CFB reaction section, the catalyst holdup will not be greater than 15% and the superficial fluid velocity will not drop below 3 m/s. With this type of arrangement, the circulation ratio of solid catalyst to acrylonitrile produced can be reduced to 100 or less. Since most of the reaction heat has been removed from the bottom part, the remaining conversion that occurs in the top part of the CFB reactor can be controlled with only a fraction of the former catalyst circulation rate that was necessary to control reaction temperatures when using the CFB reactor alone.
The primary reaction for the production of acrylonitrile consumes significant amounts of oxygen. Typically, an air stream supplies these reactants. Staged injection of the oxygen input stream to the CFB reaction section can further improve the selectivity of the reaction. In another aspect of this invention, an oxygen-containing stream, typically air, is injected at a plurality of points along the length of the CFB reaction section. Whether in gas phase or bound in a carrier material, staged oxygen injection overcomes a primary difficulty of introducing air or any oxygen-containing stream into the reaction zones which can result in a mal-distribution and locally high temperature excursions within the bubbling or circulating fluidized-bed of catalyst particles.
Staged introduction of lattice bound oxygen into either reaction zone may provide further control of temperatures within the hybrid reactor. Controlling the utilization of lattice oxygen in the ammoxidation reaction zone reduces the temperature rise resulting from the oxidation reactions since the heat generated during the oxidation of the catalyst can be removed in the regenerator. This invention can inject the oxygen carrier material instead of gas phase oxygen into the reactor. Staged introduction of the lattice bound oxygen can increase the reaction selectivity since non-selective gas phase oxidation is avoided. Furthermore, staging the introduction of particulate material and its bound oxygen at several locations along the CFB reaction section can provide a more uniform oxygen supply than side injection of gas phase oxygen. More uniform oxygen supply benefits product selectivity. Poor mixing that may result from the side injection of oxygen gas can produce some local zones of high oxygen concentration that degrade product selectivity. In this manner, the amount of gas phase oxygen at the bottom of the bubbling fluidized-bed reaction section can be reduced far below the stoichiometric amounts required for ammoxidation and the remaining part may be supplied to the hybrid reactor through a catalyst that serves as a source of lattice oxygen.
Once depleted in lattice oxygen, the catalyst may be readily regenerated by oxidation with another source of oxygen. Regeneration via oxidation may take place in a separate regenerator. Furthermore, the regeneration zone may also serve as a nitrogen generator by isolated recovery of the oxygen-depleted gas stream from the regenerator.
Accordingly, in one embodiment, this invention is a process for the ammoxidation of propylene to produce acrylonitrile. The process contacts reactants with an ammoxidation catalyst at propylene ammoxidation conditions. In the process, reactants such as propylene and ammonia contact the ammoxidation catalyst in a dense phase reaction zone. The dense phase reaction zone maintains a dense bed of catalyst at fluidized conditions, including a catalyst holdup of at least 25% at a superficial gas velocity not exceeding 1.0 m/s. Indirect heat exchange with a cooling fluid cools the dense bed. An intermediate reaction stream from an outlet end of the dense phase reaction zone along with an oxygen source pass into a circulating fluidized bed (CFB). The intermediate reaction stream contains unreacted propene. The ammoxidation of propene continues as the intermediate reaction mixture passes through the CFB reaction zone and while a catalyst holdup not greater than 15% and a superficial gas velocity of at least 3 m/s are maintained in the CFB. As a result, a reacted stream containing acrylonitrile is recovered from the CFB.
In an apparatus embodiment, this invention is an apparatus for controlling the temperature and for limiting the backmixing of catalytic reactions. The components of this apparatus are a first and a second vessel, an indirect heat exchange surface, a reducer, at least one injector, at least one separator, and a separator outlet. The first vessel defines a dense phase reaction zone having a first transverse area and at least one reactant inlet. The at least one reactant inlet receives a plurality of reactants. An indirect heat exchange surface on the interior of the first vessel indirectly cools the dense bed reaction zone. A second vessel defines a circulating fluidized bed (CFB) reaction zone having a second transverse flow area less than the first cross-sectional flow area and a reactant outlet. A reducer joins an outlet end of the first vessel with an inlet end of the second vessel and establishes a reactant and catalyst flow path from the first vessel through the reducer and the second vessel and then out of the reactant outlet. At least one injector introduces a reactant-containing stream into the reducer or the second vessel along the reactant and catalyst flow path. At least one separator communicates with the reactant outlet to separate catalyst particles from the reactant streams, and a separator outlet then delivers a reacted stream.
In a more specific apparatus embodiment; this invention is an apparatus for the ammoxidation of propene to acrylonitrile by contact with a particulate catalyst. This apparatus is composed of both a first and a second vessel, an indirect heat exchange surface, a reducer, an oxygen injector, a separator, and a separator outlet. The first vessel defines a dense phase reaction zone having a first transverse area and at least one reactant inlet. The reactant inlet receives propylene and/or ammonia reactants. Another inlet or the reactant inlet can receive air or oxygen. An indirect heat exchange surface on the interior of the first vessel indirectly cools the dense bed reaction zone. The second vessel defines a circulating fluidized bed (CFB) reaction zone having a second transverse flow area less than the first cross-sectional flow area and a reactant outlet. A reducer joins an outlet end of the first vessel with an inlet end of the second vessel and establishes a reactant and catalyst flow path from the first vessel through the reducer and the second vessel and then out the reactant outlet. At least one oxygen injector introduces an oxygen-containing stream into the reducer or second vessel along the reactant and catalyst flow path. At least one separator communicates with the reactant outlet to separate catalyst particles from the reactant stream, and a separator outlet delivers a reacted stream.
Additional objects, details, and embodiments of this invention are disclosed in the following detailed description of the invention.